Voltage variable capacitor (VVC) tuning of radio frequency circuits has been made commercially attractive by the introduction of semiconductor diodes which have been developed to the point where their use is economical and practical.
A p-n junction diode when back biased presents a capacitance that varies with bias. Such capacitors can be made to have large nominal capacitance values, have very low control current requirements, can be varied over a substantial range, and have very good electrical performance. The nature of the semiconductor diode results in very small size and freedom from mechanical effects.
Basically the diode is r-f connected across an inductor or tuned circuit. D-c coupling means is provided for applying a control voltage which can be generated remotely by a potentiometer or other electronic circuit. As reverse bias is increased the diode depletion region widens and its capacitance decreases thereby raising the circuit resonant frequency. Thus frequency is proportional to reverse bias voltage.
Since the diode is small it can be located physically at the optimum point with only an electrical connection for tuning. This greatly facilitates both electrical and mechanical circuit design.
In one use of VVC tuning, a bank of touch buttons is associated with an array of electronic latches and potentiometers. When a touch button is actuated, the associated potentiometer is connected to a source of regulated d-c voltage. The circuit array is interconnected so that when any one potentiometer is actuated all others are deactuated. Indicating lamps may be incorporated to show which of the plurality of potentiometers is in operation. The variable d-c voltage from the potentiometers is used to control the tuning of the r-f circuits in a receiver so that the touch-buttom selected potentiometer can be used as either a knob-operated remote-tuning control or a preselected frequency-control device.